The present invention relates to small air-cooled internal combustion engines, especially those utilizing a carburetor, such as engines in a snow thrower, lawn mower, rototiller, log splitter, etc. Cold temperature starting of the engine requires a more fuel-rich fuel-air mixture in the intake manifold of the engine to sustain the combustion reaction. In some engines, this is done by manually closing a choke valve in the carburetor, thereby partially choking off the air supply to the engine. As the engine warms up, the choke is no longer necessary because the increased temperatures in the engine help to sustain the combustion reaction and thus the choke is opened, allowing more air into the intake manifold. In many small engines, the choke valve is actuated manually, but may be actuated automatically, either mechanically or electrically.
Typically, during warm engine restarts, the carburetor's choke valve must remain open to start the engine and to prevent the engine from stumbling or stalling. During cold starts, if the choke valve is opened too soon, the engine may stall because the fuel-air mixture is not rich enough to sustain the reaction. Conversely, if the choke remains closed too long, the engine may also stumble and excessive hydrocarbon emissions and fouling of the spark plug can occur.
To prevent the choke valve from being in an incorrect position either during cold engine starts or warm engine restarts, various automatic choking mechanisms have been developed, including that disclosed in commonly-owned U.S. Pat. No. 7,628,387, incorporated entirely herein by reference. FIG. 1 illustrates an air/fuel mixing apparatus 5 of U.S. Pat. No. 7,628,387. Air/fuel mixing apparatus 5 includes a carburetor 10 and a solenoid 118 and is configured for use in outdoor power equipment. The carburetor 10 includes a body 14 defining an air/fuel passageway 18 along a central axis 22. A throttle lever 30 is coupled to a throttle valve (not shown) via a throttle shaft (also not shown) such that the throttle valve is pivotable about an axis substantially normal to the central axis 22 of passageway 18. Additionally, a choke lever 46 is coupled to a choke shaft (not shown), which is in turn coupled to a choke valve (not shown) that is configured to be pivotable substantially normal to central axis 22 in passageway 18 to enable restriction of air flow through passageway 18. As is set forth in detail in U.S. Pat. No. 7,628,387, throttle lever 30 and choke lever 46 each interact with respective cam surfaces to control and alter the amount of throttle and choke restriction in passageway 18.
When the engine has started and reached its normal operating temperature, a rotary solenoid 118 may be activated to further pivot choke lever 46 via a lever 122 to disengage choke lever 46 from throttle lever 30 and maintain the choke valve in a substantially opened position. A thermal switch 134 is operably coupled in circuit with the solenoid 118 and a power source 128 (e.g., a battery, a DC power source, or engine stator). The thermal switch may be surface mounted to any of the exhaust components of the engine (e.g., the muffler), or positioned in the exhaust stream of the engine (e.g., in an exhaust manifold of the engine), to detect exhaust temperature of the engine, which is indicative of the operating temperature of the engine. Thermal switch 134 is normally open, such that the solenoid 118 remains de-energized when the ambient temperature or exhaust temperature of the engine is below a predetermined value (e.g., during an initial cold start of the engine or engine restart). After the ambient or exhaust temperature reaches the predetermined value, however, the thermal switch 124 closes to energize the solenoid 118 which, in turn, pivots the choke lever 46 to place the choke valve in a substantially-opened position.
During hot restart of the engine, thermal switch 134 will be closed above a predetermined temperature value. As such, immediately upon engine starting, power is supplied to the solenoid 118 to energize the solenoid 118, which will again pivot the choke lever 46 such that the choke valve is maintained in a substantially-opened position. While such a configuration may work well for most engine applications, cold-weather applications, such as use on a snow thrower at ambient temperatures generally below 40° F., presents unique challenges. In such cold-weather applications, the engine generally needs at least a small amount of choke to adequately start, even under hot restart conditions. Air/fuel mixing apparatus 5 of U.S. Pat. No. 7,628,387 only enabled solenoid 118 to substantially open the choke valve under hot restart conditions. Accordingly, it would be advantageous to have an air/fuel mixing apparatus for use in cold-weather applications having the ability to apply a variable choke amount under such hot restart conditions.